Plasticized polyvinyl butyral (hereinafter “PVB”) is commonly used in the manufacture of polymer sheets for use as interlayers in light transmitting laminates such as safety glass or polymeric laminates. Safety glass typically refers to a transparent laminate comprised of a polyvinyl butyral sheet sandwiched between two panes of glass. Safety glass often is used to provide a transparent barrier in architectural and automotive openings. Its main function is to absorb energy, such as caused by a blow from an object, without allowing penetration through the opening, thus minimizing damage or injury to the objects or persons within an enclosed area. Additives to the sheet formulation generally include adhesion control agents (“ACA's”) to modify adhesion of the sheet to the glass so that a suitable level of adhesion can be maintained so as to prevent spalling of the glass and yet provide adequate energy absorption on impact. The interlayer sheet also may be modified to impart additional beneficial effects to safety glass such as to attenuate acoustic noise, reduce UV and/or IR light transmission, and/or enhance the aesthetic appeal of window openings.
Safety glass typically is formed by a process in which two layers of glass and a plastic interlayer, such as PVB, are assembled into a pre-press, tacked into a pre-laminate, and finished into an optically clear laminate. The assembly phase involves laying down a piece of glass, overlaying a PVB sheet, laying down a second piece of glass, and then trimming the excess PVB to the edges of the glass layers.
The plastic interlayer generally is produced by mixing PVB polymer with one or more plasticizers, and optionally with one or more other ingredients, and melt processing the mix into sheeting, which typically is collected and rolled for storage and transportation. In the lamination process for automotive windscreens, sections of PVB sheet typically are cut from the roll and these cut sections are shaped and/or stacked for assembly. A cut section then is taken from the stack and assembled in layered arrangement with a rigid substrate (e.g., glass sheet having a particular optical quality) such that a face of the rigid substrate and a face of the cut section are disposed in intimate contact and form a pre-press laminate assembly. Alternatively, this laminate assembly may be formed by interleaving multiple cut section(s) with multiple rigid sheets.
Plasticized PVB sheet, whether in roll form or in stacked form, inherently tends to stick to itself (“block”) at ambient temperatures typically encountered before and during the laminating process. Many attempts to enhance the blocking resistance of PVB have been made, including mechanical roughing of the sheet surfaces (e.g., embossing), applying a powder such as sodium bicarbonate to the sheet surfaces, and chemically or physically treating the surfaces of the PVB sheeting. Such surface treatments unfortunately often introduce undesirable handling or glass adhesion issues. In another common practice to avoid such blocking, the PVB sheeting may interleaved with another sheet material, such as polyethylene, or may be stored and transported under refrigeration, e.g., at temperatures from about 5 to about 15° C. However, for variations of standard PVB sheeting, such as PVB sheeting incorporating high plasticizer content to accomplish an enhanced noise reduction (acoustic) function, blocking may occur even under refrigeration conditions.
Additionally, it has been proposed to incorporate various blocking-resistant materials into the PVB. Incorporation of such materials into the PVB, however, may adversely affect the optical properties of the resulting laminate or the adhesive properties of the PVB sheet to glass.
Accordingly, further improved methods are needed to enhance the blocking resistance of PVB sheet, without adversely affecting optical clarity of laminates and glass adhesion properties of the resulting PVB sheet.